Maximal-ratio diversity receiving system

ABSTRACT

n RF diversity signals are routed to separate channels and heterodyned to n IF signals having the same IF frequency. The IF signals are linearly combined to provide a common IF signal from which an AGC signal is generated and is used to control the gain of each IF signal produced to assure a constant amplitude, common IF signal. This common IF signal is also used as a reference signal for a phase comparator in each channel. Each phase comparator compares the phase of its associated IF signal to the reference signal and adjusts its associated IF signal to be inphase with the other IF signals. A variable impedance diode is coupled to be responsive to each IF signal. The impedance of each diode is controlled by comparing the amplitude of the associated IF signal to the remaining (n-1) IF signals. The IF signal output of each diode is combined to provide a maximal-ratio combined IF signal for utilization in the remainder of the receiver.

United States Patent Guirak et al. Feb. 8, 1972 [54] MAXIMAL-RATIO DIVERSITY Primary Examiner-Richard Murray RECEIVING SYSTEM Attomey-C. Cornell Remsen, Jr., Walter J. Baum, Paul W. l-lemminger, Charles L. Johnson, Jr., Philip M. Bolton, Isidore 1 lnvemom REM"! G Summit; Milton Togut, Edward Goldberg and Menotti J. Lombardi, Jr.

Reicher, Parkndge, both of NJ. 73 Assignee: International Telephone and Telegraph [571 ABSTRACT CorporauomNufley, Ni n RF diversity signals are routed to separate channels and [221 if$1i$$il2fi $332112??? [21] Appl. No.: 70,624 signal from which an AGC signal is generated and is used to control the gain of each IF signal produced to assure a conmnt a}. {2%} fig fig-'33: 31s,, sed e'lil feie'fifiilgi' $.51 a 33333138153511; 58 Field of Search ..325/56, 301, 303, 305, 306; t i l g 2: g P? '3 382?? W154 156; 343/2, Z3222; IF 35d ,3 be in'hi?e"fi$'ihe 1 215 Z variable impedance diode is coupled to be responsive to each [56] Rafe cued IF signal. The impedance of each diode is controlled by com- UNITED STATES PATENTS paring the amplitude of the associated IF signal to the remaining (n-l) IF signals. The IF signal output of each diode is 3,444,471 5/1969 French ..325/305 X combined to provide a maximal rafio combined 1p signal for 3,475,688 10/1969 Masonson. ..325/305 utilizationinthe remainder fth receiver; 3,566,274 2/1971 D1 Toro ..325/305 10 Claims, 6 Drawing Figures u ,3 i ,4, I, z IF I AM i IF 1 WITH 1 C AME I be we 6 8 7 i sa VAR/ABLE slmll l cmcu/r 4 FREQUENCY v 6 PHASE aam' b IF OSCILLATOR coMPAkAmR LINE AME I souncs 27 A i mHMo/v AG I I q- DEIC7DR a VAR/A8 PHA SE smv IF 51v fi'iifflerfifi cwmmmn u/ws AM f sconce x gg l 10 SIGNAL 1 2 1 1 H w/fl} AMI? a w f I f m be comma: CIRCUIT j 1 PEAK PEAK P'AK PEAK l I F IF osrscnm osrecrQR 4251600 I QETECN AME l (-1 l8 7 e a /l |+c ca-cr a I -:1 tjg j c, Ill

' 24 R I /4 l I g ,5? Z 1 ,5, 9 l 7 l 2 a 86 43 "L Z 0r 9 IXAXIMAL-RAT/O 02 i e xf wilk 75 m OF RECEIVER PAIENTEUFEB 81972 SHEET 3 OF 6 a a m N 7 M a rf c AP? E: e r 1 c mc 0 r a ARA. E c 0M 0 N M A w A w l :7 0 Ma m a RH c s MR mm& m 0 W @o W W T 9 W A p M M \A W 6 T a! m m WQQO Q K WUEYQW0$ u$ q w w w III DIODE URlQA/7I y) 1'0 MAMPS nu M s w u T N .QN EDD E v 6 mm A mm m PATENTEDFEB 8 I972 SHEET 8 [1F 6 MAXIMAL-RATIO DIVERSITY RECEIVING SYSTEM BACKGROUND OF THE INVENTION This invention relates to radio diversity receiving systems responsive to angularly modulated carrier waves and more particularly to a radio diversity receiving system of the predetection combining type.

One of the difficulties encountered by radio systems for long distance communication is that of fading, generally regarded as resulting from the interference at the receiving system between those transmitted radio waves which have followed paths of different efiective lengths. Heretofore, this phase difficulty has been attacked by various forms of diversity systems, such as, space diversity, frequency diversity, time diversity or angle diversity systems, as fully described in U.S. Pat. No. 3,195,049.

Diversity has received widespread success especially with present day long distance troposcatter communication systems. Because of the weak, rapidly fading signals inherent to troposcatter conditions, these systems employ modulation techniques that provide a signal-to-noise enhancement, such as is obtainable with FM (frequency modulation) techniques, in conjunction with diversity reception to provide high quality, reliable communications.

One technique for operating on FM signals in a diversity receiver has been termed the signal selection technique. With this type of receiving technique, the stronger of the two signals is accepted and the weaker of the two signals is rejected. it was found that this type of receiving technique did not provide as much of an advantage as compared to predetection combining techniques, since both of the channels of a dual diversity system, or all of the channels of a multiple diversity receiving system, contribute to the combined IF (intermediate frequency) signal output resulting in an advantage in long distance scatter-type communication systems.

One form of IF predetection combining system has been termed an equal gain" combining system. In this system, the IF signals are generated to have equal frequencies and to have a phase relationship so that the IF signals can be combined inphase and at the same relative level that they are received. The output signal of the combiner, the common IF signal, is utilized to generate an automatic gain control (AGC) signal which is applied in common to the IF amplifiers of the diversity receiver to assure a constant amplitude, common IF signal at the output of the combiner.

Still another form of predetection combining system is called the maximal-ratio or ratio-squared combining system which is the most effective diversity combining system affording the greatest potential in signal reception reliability. This type of combining technique is similar to equal gain combining except for the method of controlling the gain for each predetected IF signal. Equal gain combining requires that the relative gain for each predetected IF signal be the same, where as maximal-ratio combining requires that the gain for each predetected IF signal be proportional to the signal level itself. In the resultant common IF output signal, the weaker signal is controlled to contribute a proportionally small amount of itself than does the stronger signal of the combining signal. The common AGC voltage of the equal gain combining technique is still employed in the maximal-ratio combining arrangement to maintain the correct ratio of the IF signals at the input to the combiner.

Our copending application, Ser. No. 804,175, filed Mar. 4, 1969 and assigned to the same assignee as the present application discloses therein an equal gain diversity receiving system with squelch. In this receiving system, a diode is disposed in each of the signal paths prior to combining with the diodes being under control of a squelch circuit which responds to the AGC signal and the relative carrier ratios of the two IF signals. The squelch diodes supply no attenuation when the relative carrier ratios are less than a predetermined value and the squelch diode associated with the weaker IF signal supplies substantial attenuation when one of the relative carrier ratios is equal to or greater than the predetermined value. This arrangement produces a near maximal-ratio operation.

The above-cited U.S. Pat. points out the various advantages of predetection combining techniques with the primary advantage thereof being to increase the probability that the receiver threshold is exceeded for a greater percentage of the time, thereby improving communication reliability.

SUMMARY OF THE INVENTION An object of this invention is to provide still another type of predetection combining diversity receiving system.

Another object of this invention is to provide a diversity receiving system for combining substantially inphase a plurality of FM signals employing maximal-ratio combining techniques.

Still another object of this invention is to provide a diversity receiving system of the maximal-ratio combining type which is achieved by modifying the equal gain combining with signal squelch circuit arrangement disclosed in the above-identified copending application.

A feature of this invention is the provision of a diversity receiving system of the predetection combining type comprising n sources of signals, the signals of each of the sources having random phase relation with respect to each other, where n is an integer greater than one; first means coupled to the sources to provide n IF signals each having the same frequency; n gain control means each coupled to the first means responsive to a different one of the IF signals; n variable impedance means each coupled to a different one of the gain control means; second means coupled to the gain control means to linearly combine the IF signals; third means coupled to the output of the second means, the output of each of the gain control means and the first means to vary the phase relation of the IF signals to render the IF signal inphase at the outputs of the first means; fourth means coupled to the output of the second means to produce a control signal for coupling to each of the gain control means to control the amplitude of each of the IF signals to produce a constant amplitude signal at the output of the second means; fifth means coupled to the output of each of the gain control means and each of the variable impedance means to control the impedance of each of the variable impedance means to provide square law signal addition; and sixth means coupled to each of the variable impedance means to combine the IF signals and provide a maximal-ratio combined output IF signal for the system.

BRIEF DESCRIPTION OF THE DRAWING The above mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a dual diversity receiving system employing the maximal-ratio combining technique in accordance with the principles of the present invention;

FIG. 2 is a graph of the detected carrier level differences illustrating how the control voltages for the variable impedance diodes of the DC (direct current) control circuit of FIG. 1 are produced;

FIG. 3 is a graph illustrating the dynamic impedance variation for a Solitron MS 7330 diode that may be employed as the variable impedance diodes of FIG. 1;

FIG. 4 is a graph illustrating the diversity improvement achieved by the combining techniques of the present invention;

FIG. 5 is a graph illustrating the maximal-ratio combined output variation in accordance with the principles of this invention; and

FIG. 6 is a block diagram of a predetection combining diversity receiving system in accordance with the principles of this invention having more than two diversity signals applied thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, there is illustrated therein a diversity receiving system incorporating the maximal-ratio combining technique in accordance with the principles of this invention for a dual diversity receiving system. Two RF (radio frequency) diversity signals f A and f are routed to separate diversity signal channels. The two diversity signals, f A andf will have a RF particular frequency depending upon the type of diversity technique employed. The type of diversity techniques which may produce diversity signals f and f, are fully disclosed in the above-cited patent.

The two diversity signals f A and f, are applied to two separate channels each of which includes mixer 1, variable frequency oscillator source 2 and [F amplifier 3 with AGC. The requirement of mixers 1 and 1 together with their oscillator sources 2 and 2' is to provide two [F signals f having the same frequency for application to IF amplifiers 3 and 3'. If the RF signals, f A and f are of different frequencies, as would be the case in a frequency diversity system, the frequencies and f would be different so as to provide the same IF frequency output from mixers l and 1'. The IF signals C, and C are then coupled to a phase control and AGC circuit 4 which includes therein an IF amplifier 5 for each of the signals C, and C,. The outputs of amplifiers 5 and 5' are linearly combined on conductor 5a and used as a reference signal for phase comparators 6 and 6. The common IF reference signal is coupled through IF amplifiers 7 and delay lines 8 prior to application to comparators 6. The other input of comparator 6 is coupled to the output of amplifier 3 and the other input of comparator 6' is coupled to the output of amplifier 3'. Comparaage V2 for coupling through resistor 26 to the input electrodes of variable impedance diode D2. Thus, the impedance of diode D2 is adjusted which is proportional to the difference in carrier levels (C,-C,

The following discussion will provide a mathematical proof that the combined IF signals at output 9 are combined on a square law, or a maximal-ratio basis.

Maximal-ratio combining differs from other predetection combining techniques only in the manner in which the signal gain for each diversity channel is controlled prior to combining for utilization in the remainder of the receiver.

Equation (1) represents the combined carrier-to-noise ratio for any predetection combining system with (n) diversity channels where C,, C, C,,= received carrier signal levels; N,. N, N. diversity channel noise levels and K,, K, K, diversi ty channel signal gains. I

For predetection, maximal-ratio combining, the individual channel gains (K,,) are proportional to the carrier signal level (C,) in that channel. Equation (l) for two diversity channels (n=2) reduces to:

tors 6 produce phase control signals which are coupled to oscillator sources 2 to vary the phase relation of the two IF signals produced by mixers 1 to be inphase at the output thereof for inphase combining on conductor a and at output 9 where the maximal-ratio combined IF signal is present.

Delay lines 8 are provided to compensate for phase shifts in other circuit components in the signal paths so that the reference signal and the associated IF signal are disposed in a 90 phase relationship when the IF signals are inphase. Under this condition, the phase control from comparators 6 would be zero.

AGC detector 27 is coupled to conductor 5a to provide a common AGC control signal for amplifiers 3 to ensure a constant amplitude common IF signal on conductor 50. Conductor 28 interconnects common circuit points in amplifiers 3 to compensate for circuit variations in these amplifiers and particularly circuit variations due to temperature variations.

The IF signals C, and C are also coupled to a DC control circuit 10. IF signal C, is also coupled through IF amplifier 11 and DC isolation capacitor 12 to the input electrode of variable impedance diode D1. The IF signal C is also coupled through IF amplifier l3 and DC isolation capacitor 14 to the input electrode of variable impedance diode D2. The output electrodes of diodes D1 and D2 are coupled to common output 9 through DC isolation capacitor 15.

The DC control circuit includes a reference voltage V,, coupled through resistor 16 to the output electrodes of diodes D1 and D2. In addition, positive peak detector 17 is coupled to be responsive to IF signal C, and negative peak detector 18 is coupled to be responsive to IF signal C,. The outputs of detectors 17 and 18 are algebraically combined in resistor 19 with this algebraically combined result being amplified in amplifier 20 to provide control voltage Vl which is coupled through resistor 21 to the input electrode of diode D1 to thereby adjust the impedance of diode D1 which is proportional to the difference in carrier levels (C,-C

Circuit 10 also includes positive peak detector 22 responsive to IF signal C, and negative peak detector 23 responsive to IF signal C,. The outputs of detectors 22 and 23 are algebraically combined at resistor 24 with the resultant DC voltage being amplified by amplifier 25 to produce a control volt- ;If N, =N2, which is usually the case:

No N (2) :For equal inputs, C, C equation (2) becomes:

2% Qt No N h 2 N For a complete fade of one of the carrier signals C, or C 0, equation (2) becomes:

In a reduction to practice of the receiving system of FIG. 1, diodes D1 and D2 were hot carrier diodes, such as, Solitron MS 7330 diodes, and the operating frequency of the IF signals is 70 megahertz (MHz).

The 70 MHz. series impedance of each of diodes DI and D2 is a function of the magnitude of the DC current through the diode as shown in FIG. 3. The DC current, in turn, is a function of the relative carrier signal strengths, C, and C and Va ries in such a way as to produce signal attenuation on a maximal-ratio basis.

In control circuit 10, it is essential that the DC control voltage VI and V2 be directly proportional to the difference in carrier signal levels, C,-C or C C, as shown in FIG. 2. The curves in FIG. 2 are plots of the measured normalized DC output V1 or V2 and the theoretical normalized carrier difference equation, as a function of the relative carrier level C,/C and Cz/C The close agreement of the measured and theoretical value R1 that:

and

V and V is approximately equal to 0.5 volt DC (v. DC), which is negligible in comparison to the magnitude of the reference voltage V,,=l2 v. DC.

The individual gains A 1 and A2 are adjusted so that:

Vk-Vl Therefore, with good approximation:

The choice of V,,, R and R1 depends on the desired range of operation of the dynamic diode impedances, R and R With the Solitron MS 7330 hot carrier diodes being used, see FIG. 3, they have a dynamic impedance which is closely approximated by R,;=26/I,, ohms, where 1,, is in milliamps, in the most critical diode impedance range of operation. It is essential that this diode impedance characteristic, R,,=26/I,,, be at least nearly the same for both diodes D1 and D2.

The desired range of operation of R,,, and R depends on the following:

1. The 70 MHz. source and load impedance seen by the diodes. The source impedance must be as low as possible (less than 10 ohms for the reduction to practice) and the load impedance must be as high as possible.

2. The diode impedance variation capability at 70 MHz. (see FIG. 3) which will provide the desired minimum and maximum 70 MHz. signal attenuation range.

3. The diode impedances must be sufficiently low so as not to cause phase shifts that may result in out-of-phase signal ad- WLQEL The amplifiers 20 and 25 must be capable of supplying linearly i4 v. DC.

From Equation (4);

Therefore, the minimum value of R is:

R 26/04 65 ohms Conversely, for C 0: I 0, R maximum, I 0.4

ma., and

I0 =0.4 ma

I greater than 2,000 ohms. This provides about 25 db. (decibel) maximum attenuation of the weaker can'ier signal.

The complete 70 MHz. equivalent circuit includes a source Esjstgr forgagh gfsignals C, and C, in series with the series circuit including the diode resistance R and R with the output being taken across a load resistor connected in a parallel between ground and the appropriate diode resistance.

The source resistor is less than 10 ohms which is considered negligible and not included in the following analysis and the load resistor is greater than 5,000 ohms and is not considered in the following analysis.

The contribution of C, and C to the maximal-ratio output C is determined by the relative magnitude of R and R,,,, which are, in turn, controlled by C, and C,.

C01 Rm'i'Rnz and R T C D! 02 m n? 26 m C01 C02, Rm R02: I01 I;

Therefore:

This equation reduces to equation (5).

I c m m C m or 02 1 1m +1122 2 (5) lf k,-C, and K =C,, then the maximal-ratio addition of C, and C, as specified in Equation 1) will be achieved.

The next step is to obtain an equivalent expression for that contains only the carrier signals Cl and C2.

circuit and the boundary conditions of signal operation.

The common AGC maintains the following:

C,+C =C,, constant 7 Rearranging Equation Equation (7) into a more convenient form results in It follows from this expression that:

. C C V1 =A1(C1 2) AGC'i Ci At one of the boundary conditions C 0, therefore: V1=Al C =Al (C,=C,,) (8) At this same boundary condition I 0 and from Equation (5) C,,, C,. Substituting C,,, C, and C =0 into Equation (6) Substituting Equations (3) and (4) into Equation (5) and This can be accomplished by considering the common AGC Substituting Equation (8) into Equation (9) results ifi' the desired equivalent expression.

According to Equation (7), C 5- C, is held constant by thecommon therefore:

AGC AGC Therefore, comparing Equation (5) to Equation (1 l l l and e z Accordingly, maximal-ratio action is achieved. The above analysis is predicated with the requirement that the 70 MHz. signal be sufficiently low so as not to upset the predicted dynamic impedance variations. Large 70 MHz.

signals can cause a change in diode current due to signal self-' rectification. This can be efiectively eliminated by operating the DC control circuits between large DC voltages, such as V =l 2 v. DC, in comparison to the self-rectified voltage.

In addition, large 70 MHz. signals can cause a change in the average dynamic impedance of the diode, because of the 70. MHz. voltage swing across the diode.

It has been determined empirically that 70 MHz. signal levels of 30 millivolts, root means square or less will result in little or no change in the average dynamic impedance.

Static diversity improvement characteristics have been measured employing a receiving system with an equal gain combiner and the maximal-ratio combiner of this invention.i

-At equal carrier signals C t C therefore:

At C, or C 0, the common AGC causes C or C to in- ..cr a to twise its level at the qual a etes dit sm L 21 Q Cm Q. W129.

Therefore, the maximal-ratio output, C,,,, will be 6 db. greater at the faded signal condition that at the equal signal condition.

The measured and theoretical normalized, maximal-ratio v70 The IF attenuation circuit consists of (11) hot earner diodes,

output is plotted in FIG. as a function of a relative carrier signal level.

The foregoing description and analysis has been based on a dual receiving system (F2). Referring to FIG. 6, there is disclosed therein a maximal-ratio combining arrangement for a multiple fold diversity system where n is greater than 2. As in asses FIG.- L hq'l tasteftqqygaetsisvflsetslaits t separate channels each of which contain a mixer similar to mixer l and variable frequency oscillator source 2 to provide an IF signal at the output of mixer l in each diversity channel which has the same frequency. Each diversity signal channel 5 further includes an IF amplifier with AGC similar to amplifier" 3 of FIG. I to provide a signal C, in the associated signal channel. The signals of each diversity channel are combined ,through means of emitter followers 29 and resistors 30 for coupling to amplifier 31. As in FIG. I, the IF signals are 1 linearly combined with each of the IF signals being inphase with each other. The associated phase control signals are produced in phase comparators 32 each of which receive their associated s i aLC and a reference signal derived from the common output of amplifier 31. This reference signal is coupled from amplifier 31 through amplifier 33 and, hence,

through emitter followers 34 to their associated delay lines 35. Thus, the phase control signals from phase comparators 32 will operate on the oscillator source of its associated channel to cause the IF signals to be inphase so that they may be linearly combined at the input to amplifier 31 and also at the output 36 which provides the maximal-ratio combined IF? :signal for utilization in the remainder of the receiver.

The common IF signal at the output of amplifier 31 is also coupled to AGC detector 37 to provide through emitter fol I lower 38 the common AGC signal for gain control through means of IF amplifiers with AGC in the associated diversity 'channel.

The maximal-ratio circuits 39 are provided for each diversity signal channel. Each of circuits 39 control the impedance of its associated diode Dl-Dn which is coupled to its associated signal channel through means of emitter follower 40, resistors 41 and 42 and capacitor 43. As in FIG. 1, a reference voltage -V is coupled through resistor 44 to its associated diode such as diode D1. The output of emitter follower is also coupled to positive peak detector 45. The remaining (n-l) signals are combined through the means of emitter followers 46 and resistors 47 to provide an output equal to the average amplitude of the remaining (n-l) carrier signal levels. This combined output is then coupled through amplifier 48 having a gain of V n-l. The output of amplifier 48 is then coupled to a negative peak detector 49. The DC outputs from detectors and 49 are coupled to resistor 50 for algebraic addition therein. The voltage V0 is coupled to amplifier 51 which has a gain of G1 to produce the control voltage V1 which is coupled through resistor 52 to the input electrode of the associated diode, such as diode Dl, for co ntrolof th e impedance thereof.

The signal output from each of circuits 39 is co upled through a DC isolation capacitor 53 wherein each of the as-- sociated channel signals now is C,, and are added inphase to provide a combined maximal-ratio IF signal which is applied through emitter follower 54 to amplifier 55 and, hence, to output 36.

In effect, the circuits 39 are subdivided into an IF attenua- 55 tion circuit which includes diodes Dl-Dn, two DC current limiting resistors and a DC control circuit controlling the impedance of the associated diode. One of each of these circuits is required for e%h received signal.

The maximal-ratio combiner of FIG. 6 is intended to be 0 used by a receiver with a common AGC so that the relative ratio of the signals in the receiver Rf inputs is preserved at the maximal-ratio combiner IF inputs. The operation of the maximal-ratio circuit upon each of these IF signals results in an IF signal combination on a square law basis, namely, C -lC l-C The IF attenuation circuit provides the additional IF gain control, which for each signal path is proportional to the strength of the IF signal in that signal path relative to the strength of the IF signals in the remaining (n-l) signal paths.

one diode for each IF signal path in a series-shunt arrangement, each diode being driven from a low impedance IF source into a high impedance IF load.

The range of operation of R and R is chosen so that the source resistance and the load resistance having negligible efvalue of R,,, is chosen to E Ke same as that disclosed where V negative reference voltage; R equals tlne value of hereinabove, 65 ohms, corresponding to a maximum diode the current determining resistors 44 and 52 and Vl=DC concurrent of I 0.4 ma. for the Solitron MS 7330 hot carrier trol voltage. diode. The solution to Equations (l2), (l3) and l4) are obtained The output of the IF attenuation circuit iS given by: 5 under the specific boundary conditions imposed by the received IF signals. The specific IF signal boundary conditions 1 for signal C, are: 1 1. Maximum signal C +C C,,=0 R0 R R 2. Zero signal: C,=0 1 For any of these signal boundary conditions, the receiver RBI common AGC maintains the following:

C,+C +C C,,=C,, ==Constant (l5) Consider the first boundary condition, C +C C,,=0, For the Solitron MS 7330 diode R 26/1, Therefore, where C! atamaxlmum- IS F E ti 15 t d 1 th above e ression results m ,M R 1"? '1 ,1. 4Q; Subsmu mg an slmp lfymg e xp From Equation (13) the DC control voltage is a positive is accomplished through proper diode current control by the DC control circuits.

Equation 12) M maximum.

I Vl=Al(C,)=BA1 C, =Vl I +I,, 1,, 1 (12) iwhere Vl,,,,, maximum output ofamplifier 51. w Consider the second boundary condition C,=0, where C, is where 1 1 diode DC current. completely faded.

Maximal-ratio combining is achieved, when From Equation (15 C +C C,.=C

Mimi From Equation (13), the DC voltage V1 is a negative K,= Im maximum. E 2L i .Pli'.v P-Y l:;; 'i V1 A1 C2+C3+ C" A1 CAGC V1 is proportional to C,, thereby making Co, 070mm, C,. This \/n 1 V n l V n 1 From Equation (12), when C, is a minimum, the gain K, must also be at a minimum. This occurs when I ,=0. The

The attenuator diode DC control circuit enerates the DC g minimum value of K, is zero. Substntutnng mto Equation (14) voltages V1Vn that control the IF attenuation diode DC currents. Consider the DC control circuit of FIG. 6 in diversity yleliszv. WW4 channel C,. The detected IF signal in channel C, is compared V1 .to the detected sum of the IF signals in the remaining (n-l) max. 1K5) channels. The resulting difference voltage V0 is amplified by 101:0: amplifier 51 to produce DC control voltage V1, as given by ID. .J- -A- Solving for VR yields:

C2+C3+...C i

1,571" where Al==G1B1; B1 peak detector transfer constant; G1 Therefore: DC amplifier gain of amplifier 51; VI DC control voltage; C,, 1 g C C C, IF input signals and n order of diversity. V1 llfii V1 is then applied to diode D1 through suitable currentlf Vn-1 determining resistor 52 and 44 to control the diode current; 2R (16) 1 Similar diode current control occurs for the diode in the 7 remaining (nl) channels thereby achieving the maximal- Equation (16) is equally applicable to the remaining 1F ratio performance indicated hereinabove. Signals- The amplification of the IF signals prior to peak detection is y the pp p IF Signal subscripts for the diode such that the level of the sum of the (n-l) signals is divided by rent and Vi need be changed.

. lL: 1. in amplifier 48. This prevents an unbalance in the 1 Substituting Equations (13) and (15) into equation (16) peak detector output voltage Vo due to noise, when there is a and simplifying yields Equation 17).

complete fade of all the signals. During this zero signal condition, the receiver common AGC equally increases the IF 55 AlC, 1 17 signal path gains to the extent that the IF noise levels at the Y 1 combiner inputs are of sufficient magnitude to generate DC voltage at the output of diode detectors 45 and 49. Division by s b i i Equation 17 for [m and the corresponding V Z "l Pmduces equal IF noise levels at the P detector equations for and 1 into Equation (12) and simputs, hence, equal DC voltage at their outputs resulting in a plifyi i ld E i 18) zero input to amplifier 4] and, thus, voltage V1 is maintained at zero volts DC for the zero signal condition. For example, K (18) during ager o sigal cgrgitiqnj ,.W i i i LLQ'MMM- Jill v But from Equation (15), C,+C,+C C,,=C,, con- V1: ifi'jyh; r ir) stant. Therefore K, is proportional to C,, thus satisfying the m gain control requirement for maximal-ratio combining. '7 m 'vm" J There are certain factors in the maximal-ratio combiner arwhere N,, N N N,,=combiner input IF noise levels. rangement of FIG. 6 which impose a practical limit on the For N=N,=N N,,, which is the usualcondition order of diversity (n). Specifically, these limitations are the i a I impedance levels at the maximal-ratio combining point and V1=A1 (M- M the detection efficiency of the peak detectors in circuit 39. Wn l Each IF sigrnal sees" its own diode impedance in series with the remaining (n-l diodes in shunt.

Theoretically, the maximum diode impedance is infinite. lra lq ytifiqqfluasjaam srsa i xalvalhsrst re.

If resistor 44 and 52 have a value of R, it follows that the ,75 lmfKFi Q /ZB..-.. as

the maximum shunt impedance is 2 ,0 OO/(n l in parallel with the load resistance. As n increases the maximum shunt impedance decreases, thereby increasing the loss across diode D1 for the signal C1 in the channel carrying Cl. This, in turn, introduces errors in maximal-ratio signal combining. it is estimated that n=6 is a practical limit to this technique.

- The DC control voltage V1, as given by Equation (l3), shows that the sum of the (n-l) signals term is:

As (n) increases the magnitude of the above expression decreases; thus, V1 is less sensitive to changes in C +C C,, for large values of n and more susceptible to other circuit variations.

While we have described above the principles of our invention in connection with specific apparatus, it is to be more clearly understood that this description is made only by way of example and not as a limitation to, the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

1. A diversity receiving system of the predetection combining type comprising:

n sources of signals, the signals of each of said sources having random phase relation with respect to each other, where n is an integer greater than one;

first means coupled to said sources to provide it intermediate frequency signals each having the same frequenn gain control means each coupled to said first means responsive to a different one of said intermediate frequency signals;

n variable impedance means each coupled to a different one of said gain control means;

second means coupled to said gain control means to linearly combine said intermediate frequency signals;

third means coupled to the output of said second means, the output of each of said gain control means and said first means to vary the phase relation of said intermediate frequency signals to render said intermediate frequency signals inphase at the outputs of said first means;

fourth means coupled to the output of said second means to produce a control signal for coupling to each of said gain control means to control the amplitude of each of said intermediate frequency signals to produce a constant amplitude signal at the output of said second means; fifth means coupled to the output of each of said gain control means and each of said variable impedance means to control the impedance of each of said variable impedance means to provide square law signal addition; and sixth means coupled to each of said variable impedance means to combine said intermediate frequency signals and provide a maximal-ratio combined output intermediate frequency signal for said system. 2. A system according to claim I, wherein each of said gain control means includes an intermediate frequency amplifier with automatic gain control means; and further including a conductor interconnecting each of said amplifiers to compensate for variations therein. 3. A system according to claim 1, wherein each of said variable impedance means includes a diode. 4. A system according to claim 1, wherein said third means includes n phase comparators each coupled to the output of a different one of said gain control means and said second means, the outputs of each of said phase comparators being coupled to said first means to vary the phase relation of said intermediate frequency signals to render said intermediate frequency signals inphase at the outputs of said first means.

5. A system according to claim 1, wherein said first means includes n mixer means each coupled to a different one of said sources, and n variable frequency oscillator means each coupled to a different one of said mixer means, each of said oscillator means being controlled by said third means to vary the phase relation of said intermediate frequency signals to render said intermediate frequency signals inphase at the outputs of said mixer means. 6. A system according to claim 1, wherein n is equal to two; said variable impedance means includes a first diode having its input electrode coupled to the output of one of said gain control means and its output electrode coupled to said sixth means, and a second diode having its input electrode coupled to the output of the other of said gain control means and its output electrodes coupled to said sixth means; and said fifth means includes a reference voltage coupled to the output electrodes of each of said first and second diodes, seventh means coupled to the output of both of said gain control means to produce a first voltage equal to the algebraic addition of the positive peak of one of said intermediate frequency signals and the negative peak of the other of said intermediate frequency signals, said first voltage being coupled to the input electrode of said first diode, and eight means coupled to the output of both of said gain control means to produce a second voltage equal to the algebraic addition of the positive peak of said other of said intermediate frequency signal and the negative peak of said one of said intermediate frequency signals, said second voltage being coupled to the input electrode of said second diode, 7. A system according to claim 6, wherein said seventh means includes a first positive peak detector coupled to one of said gain control means, a first negative peak detector coupled to the other of said gain control means, a first resistor coupled between the outputs of said first positive and negative peak detectors, and a first amplifier coupled to said first resistor to provide said first voltage; and said eighth means includes a second positive peak detector coupled to said other of said gain control means, a second negative peak detector coupled to said one of said gain control means, a second resistor coupled between the outputs of said second positive and negative peak detectors, and a second amplifier coupled to said second resistor to provide said second voltage. 8. A system according to claim 1, wherein n is equal to an integer greater than two; each of said variable impedance means including a diode having its input electrode coupled to a different one of said gain control means and its output electrode coupled to said sixth means; and said fifth means includes n maximal-ratio circuits each coupled to a different one of said diodes and the output of all of said gain control means to control the impedance of the associated one of said diodes to provide square law signal addition. 9. A system according to claim 8, wherein each of said maximal-ratio circuits includes a reference voltage coupled to the output electrode of the associated one of said diodes, and seventh means coupled to the output of all of said gain control means to produce a control voltage equal to the algebraic addition of the positive peak of one of said insaid intermediate frequency signals,

a first amplifier having a predetermined gain coupled to the output of said eighth means,

a negative peak detector coupled to the output of said first amplifier,

a resistor coupled between the output of said positive and negative peak detector, and

a second amplifier having a given gain coupled to said resistor to provide said control voltage for coupling to the input electrode of the associated one of said diodes. 

1. A diversity receiving system of the predetection combining type comprising: n sources of signals, the signals of each of said sources having random phase relation with respect to each other, where n is an integer greater than one; first means coupled to said sources to provide n intermediate frequency signals each having the same frequency; n gain control means each coupled to said first means responsive to a different one of said intermediate frequency signals; n variable impedance means each coupled to a different one of said gain control means; second means coupled to said gain control means to linearly combine said intermediate frequency signals; third means coupled to the output of said second means, the output of each of said gain control means and said first means to vary the phase relation of said intermediate frequency signals to render said intermediate frequency signals inphase at the outputs of said first means; fourth means coupled to the output of said second means to produce a control signal for coupling to each of said gain control means to control the amplitude of each of said intermediate frequency signals to produce a constant amplitude signal at the output of said second means; fifth means coupled to the output of each of said gain control means and each of said variable impedance means to control the impedance of each of said variable impedance means to provide square law signal addition; and sixth means coupled to each of said variable impedance means to combine said intermediate frequency signals and provide a maximal-ratio combined output intermediate frequency signal for said system.
 2. A system according to claim 1, wherein each of said gain control means includes an intermediate frequency amplifier with automatic gain control means; and further including a conductor interconnecting each of said amplifiers to compensate for variations therein.
 3. A system according to claim 1, wherein each of said variable impedance means includes a diode.
 4. A system according to claim 1, wherein said third means includes n phase comparators each coupled to the output of a different one of said gain control means and said second means, the outputs of each of said phase comparators being coupled to said first means to vary the phase relation of said intermediate frequency signals to render said intermediate frequency signals inphase at the outputs of said first means.
 5. A system according to claim 1, wherein said first means includes n mixer means each coupled to a different one of said sources, and n variable frequency oscillator means each coupled to a different one of said mixer means, each of said oscillator means being controlled by said third means to vary the phase relation of said intermediate frequency signals to render said intermediate frequency signals inphase at the outputs of said mixer means.
 6. A system according to claim 1, wherein n is equal to two; said variable impedance means includes a first diode having its input electrode coupled to the output of one of said gain control means and its output electrode coupled to said sixth means, and a second diode having its input electrode coupled to the output of the other of said gain control means and its output electrodes coupled to said sixth means; and said fifth means includes a reference voltage coupled to the output electrodes of each of said first and second diodes, seventh means coupled to the output of both of said gain control means to produce a first voltage equal to the algebraic addition of the positive peak of one of said intermediate frequency signals and the negative peak of the other of said intermediate frequency signals, said first voltage being coupled to the input electrode of said first diode, and eighth means coupled to the output of both of said gain control means to produce a second voltage equal to the algebraic addition of the positive peak of said other of said intermediate frequency signal and the negative peak of said one of said intermediate frequency signals, said second voltage being coupled to the input electrode of said second diode.
 7. A system according to claim 6, wherein said seventh means includes a first positive peak detector coupled to one of said gain control means, a first negative peak detector coupled to the other of said gain control means, a first resistor coupled between the outputs of said first positive and negative peak detectors, and a first amplifier coupled to said first resistor to provide said first voltage; and said eighth means includes a second positive peak detector coupled to said other of said gain control means, a second negative peak detector coupled to said one of said gain control means, a second resistor coupled between the outputs of said second positive and negative peak detectors, and a second amplifier coupled to said second resistor to provide said seconD voltage.
 8. A system according to claim 1, wherein n is equal to an integer greater than two; each of said variable impedance means including a diode having its input electrode coupled to a different one of said gain control means and its output electrode coupled to said sixth means; and said fifth means includes n maximal-ratio circuits each coupled to a different one of said diodes and the output of all of said gain control means to control the impedance of the associated one of said diodes to provide square law signal addition.
 9. A system according to claim 8, wherein each of said maximal-ratio circuits includes a reference voltage coupled to the output electrode of the associated one of said diodes, and seventh means coupled to the output of all of said gain control means to produce a control voltage equal to the algebraic addition of the positive peak of one of said intermediate frequency signals and the negative peak of a predetermined value of the remaining (n-1) of said intermediate frequency signal, said control voltage being coupled to the input electrode of the associated one of said diodes.
 10. A system according to claim 9, wherein said seventh means includes a positive peak detector coupled to the output of one of said gain control means, eighth means coupled to the output of the remaining (n-1) of said gain control means to provide a signal equal to the average value of the remaining (n-1) of said intermediate frequency signals, a first amplifier having a predetermined gain coupled to the output of said eighth means, a negative peak detector coupled to the output of said first amplifier, a resistor coupled between the output of said positive and negative peak detector, and a second amplifier having a given gain coupled to said resistor to provide said control voltage for coupling to the input electrode of the associated one of said diodes. 